Lithium Niobate Wafers
Lithium niobate (LiNbO3 or LN) is one of the most versatile and well-developed active optical materials. The crystal finds wide application in electro-optics, acousto-optics, nonlinear optics and guided-wave optics. Attractive fundamental properties of lithium niobate include: wide transparency range, high electro-optic and nonlinear optic coefficients, very high electro-mechanical coupling coefficients, and chemical and mechanical stability.
G&H offer a variety of sizes and thickness with wafer diameters ranging from 75 to 125 mm. Standard thicknesses are 0.5 mm, 0.7 mm and 1.0 mm. The standard orientations are x-axis and z-axis growth. Lithium niobate is available in two compositions: congruent and 5.0 mol % magnesium doped.
Our experience growing LiNbO3 dates back to 1967, producing the first commercially available material. Since that time we have constantly refined the growth and material processing to be able to produce the highest quality, most consistent LiNbO3. From proprietary growth stations to dedicated wafer production machinery, we are able to meet the most demanding quality and quantity requirements.
Congruent lithium niobate works well for IR light sources. Magnesium doping improves the resistance to photorefractive damage in the visible wavelengths. While the majority of lithium niobate wafers are double side polished, single side polished wafers are also available. Typical applications for lithium niobate are waveguides, low pass filters, isolators and Wollaston prisms.
Over the past 50 years, G&H has developed proprietary growth stations and patented formulas to yield the highest quality LiNbO3 wafers. High purity powders are measured to assure consistent composition. Computers monitor the growth process and ensure all facets are properly managed and recorded. Thermal gradients inside the furnaces are tightly controlled to minimize stresses and strain on the crystal. This attention to detail has been the cornerstone of our commitment to making the best material possible.
Many steps are required to take an as-grown crystal and shape it into its final form. An optical wafer will go through the following processing steps:
- Edge Grinding
- Inspection and Measurement
As-grown lithium niobate boules will have randomly oriented ferroelectric domains throughout the material. The poling process aligns the ferroelectric domains along the z-axis. The boules are heated beyond their Curie temperature (1142 [Symbol]C) and an electric field is applied across the z-axis. As the boule is cooled, the ferroelectric domains are locked in-line with the z-axis and the crystal becomes single domain.
After poling the boule is shaped into a cylinder. Diamond tools are used to crop the ends of the boule and grind the outside into a cylinder. This process sets the diameter of the wafers. X-rays are used to orient the crystal axis. More diamond tools are used to grind the major and minor orientation flats. The crystal orientation is maintained to better than 6 minutes.
Individual wafers are cut from the cylindrical boule. X-rays are used to align the wafer face. An inner diameter saw is used to cut individual wafers. Critical cutting parameters monitored include feed rate, indexing, and blade deflection.
A radius profile is ground on the outside edge of the wafer. The curved profile makes the wafer more resistant to cracking and chipping during processing. Our edge grinders are state-of-the-art automated machines with a cassette-to-cassette operation and wafer thickness verification.
Lapping is a double-sided grinding process that removes any subsurface damage caused by the slicing process. After lapping, the wafers will have a more uniform thickness and be much flatter. The wafers then go through a chemical etching process that removes stresses in the crystal built-up by previous processing.
Single sided and double sided polishing are available. Customized polishing plates and carriers allow for maximum productivity while maintaining surface quality. The typical finish of our polished wafers is Ra < 0.2 nm.
Inspection and measurement
Finished wafers are 100% visually inspected for defects including scratches, digs, voids, inclusions, and grain boundaries. Various optical techniques are used to characterize the stress and strain in the wafer. After inspection, in a cleanroom environment, the wafers are packed and sealed for shipment.
APPLICATIONS OF LN WAFERS
Isolators, low pass filters, ophthalmic surgery systems, use in Pockels cells, Q-switching, range-finders, target designators, waveguides, Wollaston prisms